1. Field of the Invention
The invention relates in general, to video moire reduction, and in particular, to a method and device for adaptively defocusing the electron beam size in dependence on the incoming video format.
2. Description of the Prior Art
Moire is a common word for patterns which look like waves on water. An example in the real world is the interference between two fences on a bridge. Moire appears when two sampling processes don't match each other and there is no sufficient filtering (interpolation/decimation).
The description of moire in monitors is complex. The origin is the uncorrelated sampling process between the video format at the electrical side, and the two-dimensional sampling by the shadow mask at the mechanical-optical side. In addition, the scan line frequency and its higher harmonics as they interfere with the shadow mask are responsible for so-called scan line moire. Scan line moire is most noticeable when the picture content has constant luminance and no picture details.
Although high resolution video formats and high monitor bandwidth are desirable for sharp and detailed images, the visibility of moire usually increases with both of these. Reducing scan line moire is possible by influencing the shape and width of the electron beam itself, but unfortunately, the reduction of moire by this method and preservation of resolution is a contradiction.
For standard monitors where there is a fixed horizontal and vertical scan frequency, moire suppression is achieved by choosing a suitable mask pitch (mask sampling frequency .about.1/mask pitch) in the horizontal and vertical directions. This is not possible for multi-sync high resolution monitors having large screens which are able to display various video graphics formats. These monitors are optimized for the highest resolution video formats for an electron beam having a very small spot size. In these types of monitors, even if the pitch size is chosen to optimize the reduction of moire for the high resolution video format, moire will still appear at lower resolution video formats. Also, taking into account that the customer is allowed to change the window size by about 15%, which changes the distance between pixels, an optimal pitch cannot be found for several scan modes.
European Patent Application EPA 707,300, corresponding to U.S. Pat. No. 5,747,933, describes a moire detector for controlling the electron beam size in these multi-sync monitors. One of it's main components is an adaptive band-pass filter. The center frequency of the band-pass filter is adjusted to a moire frequency. In the case where the incoming video signal is at the moire frequency, or where there are fine details in the video image which match the moire-frequency, the electron beam is defocused, thus losing resolution in the video image.